As used herein, the term "rotary extruder" refers to an extruder having a barrel section with an eccentric bore, and a rotor that comprises a substantially cylindrical plasticizing surface rotatably mounted within the bore. Polymeric resin, typically in the form of pellets, flakes or powder, is introduced into a clearance zone between the rotor and the interior surface of the barrel through a feed port. The resin is then plasticized and delivered to a discharge port as the rotor rotates within the stationary barrel. Extruders of this general type, which differ substantially from conventional rotating screw extruders, have previously been disclosed, for example, in U.S. Pat. Nos. 3,880,564; 4,012,477; 4,501,543; and 4,813,863.
In order to successfully plasticize thermoplastic resin within a rotary extruder, the rotor and barrel portions of the extruder are desirably maintained at a temperature above the melting point of the polymer being plasticized. In the past, rotary extruders have been preheated by the use of peripherally spaced cartridge heaters disposed in bores extending longitudinally through the barrel opposite but parallel to the plasticizing surface of the rotor.
In U.S. Pat. No. 4,887,907, we disclosed apparatus for internally cooling the portion of the rotor adjacent to the bearings to reduce bearing temperature and increase bearing life. Such a feature is especially desirable when extruding resins having high melting points. In that disclosure, however, a dead air space was provided to insulate the middle section of the rotor from heat transfer between the rotor surface and the recirculated coolant.
In some instances, however, difficulties have been encountered in preheating the rotary extruder and in stabilizing the temperature of the rotor surface. These difficulties are especially noticeable at the start of a production run, before any significant heat buildup occurs from shearing polymer within the extruder.
A rotary extruder is therefore needed that is adapted for use in those applications where it is desired to selectively increase the temperature of the plasticizing surface of the rotor.